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  算法学习之二叉树的遍历
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  <h2 id="二叉树定义"><a href="#二叉树定义" class="headerlink" title="二叉树定义"></a>二叉树定义</h2><p>二叉树是每个节点最多有两个子树的树结构，二叉树的子树有左右之分，次序不能颠倒。</p>
<a id="more"></a>
<h2 id="二叉树结构"><a href="#二叉树结构" class="headerlink" title="二叉树结构"></a>二叉树结构</h2><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">TreeNode</span></span>&#123;</span><br><span class="line">    <span class="keyword">int</span> val;</span><br><span class="line">    TreeNode left;</span><br><span class="line">    TreeNode right;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h2 id="二叉树创建"><a href="#二叉树创建" class="headerlink" title="二叉树创建"></a>二叉树创建</h2><p>二叉树递归方法创建</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> TreeNode <span class="title">createTree</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    Scanner scan = <span class="keyword">new</span> Scanner(System.in);</span><br><span class="line">    TreeNode node = <span class="keyword">new</span> TreeNode();</span><br><span class="line">    <span class="keyword">if</span> (scan.hasNextInt()) &#123;</span><br><span class="line">        <span class="comment">// 判断输入的是否是整数</span></span><br><span class="line">        <span class="keyword">int</span> i = scan.nextInt();</span><br><span class="line">        node.val = i;</span><br><span class="line">        node.left = createTree();</span><br><span class="line">        node.right = createTree();</span><br><span class="line">    &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">        <span class="comment">// 输入的空节点</span></span><br><span class="line">        System.out.println(<span class="string">"空节点！"</span>);</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">null</span>;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">return</span> node;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h2 id="二叉树遍历"><a href="#二叉树遍历" class="headerlink" title="二叉树遍历"></a>二叉树遍历</h2><h3 id="先序遍历-前序遍历"><a href="#先序遍历-前序遍历" class="headerlink" title="先序遍历(前序遍历)"></a>先序遍历(前序遍历)</h3><p>按照“根节点-左子树-右子树”的顺序进行访问。</p>
<h4 id="递归遍历"><a href="#递归遍历" class="headerlink" title="递归遍历"></a>递归遍历</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">pre_out</span><span class="params">(TreeNode node)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    <span class="keyword">if</span>(node == <span class="keyword">null</span>)</span><br><span class="line">        <span class="keyword">return</span>;</span><br><span class="line">    System.out.print(node.val + <span class="string">" "</span>);</span><br><span class="line">    pre_out(node.left);</span><br><span class="line">    pre_out(node.right);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h4 id="非递归遍历"><a href="#非递归遍历" class="headerlink" title="非递归遍历"></a>非递归遍历</h4><p>对于任一节点P，<br>1）输出节点P，然后将其入栈，再看P的左孩子是否为空；<br>2）若P的左孩子不为空，则置P的左孩子为当前节点，重复1）的操作；<br>3）若P的左孩子为空，则将栈顶节点出栈，但不输出，并将出栈节点的右孩子置为当前节点，看其是否为空；<br>4）若不为空，则循环至1）操作；<br>5）如果为空，则继续出栈，但不输出，同时将出栈节点的右孩子置为当前节点，看其是否为空，重复4）和5）操作；<br>6）直到当前节点P为NULL并且栈空，遍历结束。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">print_tree_stack_pre</span><span class="params">(TreeNode node)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    Stack&lt;TreeNode&gt; stack = <span class="keyword">new</span> Stack&lt;&gt;();</span><br><span class="line">    TreeNode cur = node;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">while</span> (!stack.isEmpty() || cur != <span class="keyword">null</span>)</span><br><span class="line">    &#123;</span><br><span class="line">        System.out.print(cur.val + <span class="string">" "</span>);</span><br><span class="line">        stack.push(cur);</span><br><span class="line">        cur = cur.left;</span><br><span class="line"></span><br><span class="line">        <span class="keyword">while</span> (!stack.isEmpty() &amp;&amp; cur == <span class="keyword">null</span>)</span><br><span class="line">        &#123;</span><br><span class="line">            cur = stack.pop();</span><br><span class="line">            cur = cur.right;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>另一种方法</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">preOrderUnRecur</span><span class="params">(Node head)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">if</span> (head != <span class="keyword">null</span>) &#123;</span><br><span class="line">        Stack&lt;Node&gt; stack = <span class="keyword">new</span> Stack&lt;&gt;();</span><br><span class="line">        stack.push(head);</span><br><span class="line">        <span class="keyword">while</span> (!stack.isEmpty()) &#123;</span><br><span class="line">            Node node = stack.pop();</span><br><span class="line">            System.out.print(node.value + <span class="string">" "</span>);</span><br><span class="line">            <span class="keyword">if</span> (node.right != <span class="keyword">null</span>)</span><br><span class="line">                stack.push(node.right);</span><br><span class="line">            <span class="keyword">if</span> (node.left != <span class="keyword">null</span>)</span><br><span class="line">                stack.push(node.left);</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">    System.out.println();</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="中序遍历"><a href="#中序遍历" class="headerlink" title="中序遍历"></a>中序遍历</h3><p>中序遍历——按照“左子树-根节点-右子树”的顺序进行访问。</p>
<h4 id="递归遍历-1"><a href="#递归遍历-1" class="headerlink" title="递归遍历"></a>递归遍历</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">mid_out</span><span class="params">(TreeNode node)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">if</span>(node == <span class="keyword">null</span>)</span><br><span class="line">        <span class="keyword">return</span>;</span><br><span class="line">    mid_out(node.left);</span><br><span class="line">    System.out.print(node.val + <span class="string">" "</span>);</span><br><span class="line">    mid_out(node.right);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h4 id="非递归遍历-1"><a href="#非递归遍历-1" class="headerlink" title="非递归遍历"></a>非递归遍历</h4><p>对于任一节点P，<br>1）若P的左孩子不为空，则将P入栈并将P的左孩子置为当前节点，然后再对当前节点进行相同的处理；<br>2）若P的左孩子为空，则输出P节点，而后将P的右孩子置为当前节点，看其是否为空；<br>3）若不为空，则重复1）和2）的操作；<br>4）若为空，则执行出栈操作，输出栈顶节点，并将出栈的节点的右孩子置为当前节点，看起是否为空，重复3）和4）的操作；<br>5）直到当前节点P为NULL并且栈为空，则遍历结束。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">print_tree_stack_mid</span><span class="params">(TreeNode node)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    Stack&lt;TreeNode&gt; stack = <span class="keyword">new</span> Stack&lt;&gt;();</span><br><span class="line">    TreeNode cur = node;</span><br><span class="line">    <span class="keyword">while</span>(!stack.isEmpty() || cur != <span class="keyword">null</span>)</span><br><span class="line">    &#123;</span><br><span class="line">        stack.push(cur);</span><br><span class="line">        cur = cur.left;</span><br><span class="line">        <span class="keyword">while</span>(!stack.isEmpty() &amp;&amp; cur == <span class="keyword">null</span>)</span><br><span class="line">        &#123;</span><br><span class="line">            cur = stack.pop();</span><br><span class="line">            System.out.print(cur.val + <span class="string">" "</span>);</span><br><span class="line">            cur = cur.right;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>另一种方法</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">inOrderUnRecur</span><span class="params">(Node head)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">if</span> (head != <span class="keyword">null</span>) &#123;</span><br><span class="line">        Stack&lt;Node&gt; stack = <span class="keyword">new</span> Stack&lt;&gt;();</span><br><span class="line">        <span class="keyword">while</span> (!stack.isEmpty() || head != <span class="keyword">null</span>) &#123;</span><br><span class="line">            <span class="keyword">if</span> (head != <span class="keyword">null</span>) &#123;</span><br><span class="line">                stack.push(head);</span><br><span class="line">                head = head.left;</span><br><span class="line">            &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">                head = stack.pop();</span><br><span class="line">                System.out.print(head.value + <span class="string">" "</span>);</span><br><span class="line">                head = head.right;</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">        System.out.println();</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="后序遍历"><a href="#后序遍历" class="headerlink" title="后序遍历"></a>后序遍历</h3><p>后序遍历——按照“左子树-右子树-根节点”的顺序进行访问</p>
<h4 id="递归遍历-2"><a href="#递归遍历-2" class="headerlink" title="递归遍历"></a>递归遍历</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">after_out</span><span class="params">(TreeNode node)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">if</span>(node == <span class="keyword">null</span>)</span><br><span class="line">        <span class="keyword">return</span>;</span><br><span class="line">    after_out(node.left);</span><br><span class="line">    after_out(node.right);</span><br><span class="line">    System.out.print(node.val + <span class="string">" "</span>);</span><br><span class="line"></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h4 id="非递归遍历-2"><a href="#非递归遍历-2" class="headerlink" title="非递归遍历"></a>非递归遍历</h4><p>对于任一节点P，<br>1）先将节点P入栈；<br>2）若P不存在左孩子和右孩子，或者P存在左孩子或右孩子，但左右孩子已经被输出，则可以直接输出节点P，并将其出栈，将出栈节点P标记为上一个输出的节点，再将此时的栈顶结点设为当前节点；<br>3）若不满足2）中的条件，则将P的右孩子和左孩子依次入栈，当前节点重新置为栈顶结点，之后重复操作2）；<br>4）直到栈空，遍历结束。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">print_tree_stack_after</span><span class="params">(TreeNode node)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    Stack&lt;TreeNode&gt; stack = <span class="keyword">new</span> Stack&lt;&gt;();</span><br><span class="line">    TreeNode cur = node;</span><br><span class="line">    TreeNode pre = <span class="keyword">null</span>;</span><br><span class="line">    stack.push(cur);</span><br><span class="line">    <span class="keyword">while</span>(!stack.isEmpty())</span><br><span class="line">    &#123;</span><br><span class="line">        <span class="keyword">if</span>((cur.left == <span class="keyword">null</span> &amp;&amp; cur.right == <span class="keyword">null</span>) || (pre != <span class="keyword">null</span> &amp;&amp; (pre == cur.left || pre == cur.right)))</span><br><span class="line">        &#123;</span><br><span class="line">            System.out.print(cur.val + <span class="string">" "</span>);</span><br><span class="line">            pre = stack.pop();</span><br><span class="line">            <span class="keyword">if</span>(!stack.isEmpty())</span><br><span class="line">                cur = stack.peek();</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">else</span></span><br><span class="line">        &#123;</span><br><span class="line">            <span class="keyword">if</span>(cur.right != <span class="keyword">null</span>)</span><br><span class="line">            &#123;</span><br><span class="line">                stack.push(cur.right);</span><br><span class="line">            &#125;</span><br><span class="line">            <span class="keyword">if</span>(cur.left != <span class="keyword">null</span>)</span><br><span class="line">            &#123;</span><br><span class="line">                stack.push(cur.left);</span><br><span class="line">            &#125;</span><br><span class="line">            cur = stack.peek();</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>另一种方法</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">posOrderUnRecur1</span><span class="params">(Node head)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">if</span>(head != <span class="keyword">null</span>)&#123;</span><br><span class="line">        Stack&lt;Node&gt; stack1 = <span class="keyword">new</span> Stack&lt;&gt;();</span><br><span class="line">        Stack&lt;Node&gt; stack2 = <span class="keyword">new</span> Stack&lt;&gt;();</span><br><span class="line">        stack1.push(head);</span><br><span class="line">        <span class="keyword">while</span> (!stack1.isEmpty())&#123;</span><br><span class="line">            head = stack1.pop();</span><br><span class="line">            stack2.push(head);</span><br><span class="line">            <span class="keyword">if</span>(head.left != <span class="keyword">null</span>)&#123;</span><br><span class="line">                stack1.push(head.left);</span><br><span class="line">            &#125;</span><br><span class="line">            <span class="keyword">if</span>(head.right != <span class="keyword">null</span>)&#123;</span><br><span class="line">                stack1.push(head.right);</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">while</span> (!stack2.isEmpty()) &#123;</span><br><span class="line">            System.out.print(stack2.pop().value + <span class="string">" "</span>);</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">    System.out.println();</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">posOrderUnRecur2</span><span class="params">(Node h)</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">if</span> (h != <span class="keyword">null</span>) &#123;</span><br><span class="line">        Stack&lt;Node&gt; stack = <span class="keyword">new</span> Stack&lt;Node&gt;();</span><br><span class="line">        stack.push(h);</span><br><span class="line">        Node c = <span class="keyword">null</span>;</span><br><span class="line">        <span class="keyword">while</span> (!stack.isEmpty()) &#123;</span><br><span class="line">            c = stack.peek();</span><br><span class="line">            <span class="keyword">if</span> (c.left != <span class="keyword">null</span> &amp;&amp; h != c.left &amp;&amp; h != c.right) &#123;</span><br><span class="line">                stack.push(c.left);</span><br><span class="line">            &#125; <span class="keyword">else</span> <span class="keyword">if</span> (c.right != <span class="keyword">null</span> &amp;&amp; h != c.right) &#123;</span><br><span class="line">                stack.push(c.right);</span><br><span class="line">            &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">                System.out.print(stack.pop().value + <span class="string">" "</span>);</span><br><span class="line">                h = c;</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">    System.out.println();</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="层次遍历"><a href="#层次遍历" class="headerlink" title="层次遍历"></a>层次遍历</h3><p>即从左到右输出节点的值</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">print_tree_queue</span><span class="params">(TreeNode node)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">    Queue&lt;TreeNode&gt; queueList = <span class="keyword">new</span> LinkedList&lt;&gt;();</span><br><span class="line">    queueList.offer(node);</span><br><span class="line">    <span class="keyword">while</span>(!queueList.isEmpty())</span><br><span class="line">    &#123;</span><br><span class="line">        TreeNode n = queueList.poll();</span><br><span class="line">        System.out.print(n.val + <span class="string">" "</span>);</span><br><span class="line">        <span class="keyword">if</span>(n.left != <span class="keyword">null</span>)</span><br><span class="line">            queueList.offer(n.left);</span><br><span class="line">        <span class="keyword">if</span>(n.right != <span class="keyword">null</span>)</span><br><span class="line">            queueList.offer(n.right);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>






 
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